Electron tubes employing a hollow magnetron injected beam and magnetic field reversal focusing

ABSTRACT

Electron beam tubes are disclosed which employ a magnetron injection electron gun for projecting a hollow beam of electrons axially of the tube and generally parallel to an axially directed beam-focusing magnetic field. The axial direction of the beamfocusing magnetic field is reversed abruptly within the beam path and an electrode, such as a gate, drift control electrode, slow wave circuit, or cavity resonator, is disposed in the field reversal region to obtain enhanced electronic interaction with the beam for a given electrical potential established on the interaction electrode. Use of this feature leads to improved high-power switch tubes, traveling wave tubes, klystrons, and RF delay tubes.

United States Patent [72] Inventor Robert M. Phillips Redwood City, Calif.

[21] Appl. No. 19,173

[22] Filed Mar. 13,1970

[45] Patented Oct. 19, 1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] ELECTRON TUBES EMPLOYING A HOLLOW MAGNETRON INJECTED BEAM AND MAGNETIC FIELD REVERSAL FOCUSING 9 Claims, 28 Drawing Figs.

52 us. Cl sis/3.5,

[51] Int. Cl H01j 25/34 [50] Field of Search... 313/84;

[56] References Cited UNITED STATES PATENTS 2,300,052 10/1942 Lindenblad 315/36 X 2,632,130 3/1953 l-lull 315/36 2,652,512 9/1953 Hollenberg.. 3 l5/3.6

Primary ExaminerHerman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. AttorneysStanley Z. Cole and Gerald M. Moore ABSTRACT: Electron beam tubes are disclosed which employ a magnetron injection electron gun for projecting a hollow beam of electrons axially of the tube and generally parallel to an axially directed beam-focusing magnetic field. The axial direction of the beam-focusing magnetic field is reversed abruptly within the beam path and an electrode, such as a gate, drift control electrode, slow wave circuit, or cavity resonator, is disposed in the field reversal region to obtain enhanced electronic interaction with the beam for a given electrical potential established on the interaction electrode. Use of this feature leads to improved high-power switch tubes, traveling wave tubes, klystrons, and RF delay tubes.

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. INVENTOR. 7 ROBERT M. PHILLIPS ATTORNEY ELECTRON TUBES EMPLOYING A HOLLOW MAGNETRON INJECTED BEAM AND MAGNETIC FIELD REVERSAL F OCUSING DESCRIPTION OF THE PRIOR ART Heretofore, microwave beam tubes have been reverse field focused with an axially directed magnetic field. However, in these tubes the magnetic field threaded through the cathode emitter surface such that the individual electrons of the beam each spiraled around a magnetic flux line which threaded through the cathode emitter. In this prior tube, the purpose of the reverse field focusing was to allow a smaller magnet to be employed for focusing the beam over a certain predetermined beam path. After the magnetic field reversal, the electrons continued to have essentially the same axial velocity as they had before the field reversal. 1

In another prior art tube, a hollow beam gun has been employed for projecting a hollow beam of electrons through a slow-wave microwave circuit. A period magnetic field focusing structure was provided along the path of the hollow beam. The magnetic beam-focusing field was characterized by a succession of transversely directed magnetic field regions of alternating polarity which caused the angular beam to rotationally undulate witha period preferably corresponding to the period of wave energy on a microwave interaction circuit through which the hollow beam was projected. Such a prior art tube is described and claimed in US. Pat. No. 3,013,173 issued Dec. 12, I96] and assigned to the same assignee as the present invention.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of improved electron beam tubes employing hollow magnetron-injected beams.

One feature of the present invention is the provision, in a magnetron-injected beam tube, of means for reversing the axial direction of the beam-focusing magnetic field to define a reversed field region and including an interaction electrode disposed within the reversed field region of the beam path for electronic interaction with the beam in the reversed field region, whereby electrical potentials established on the interaction electrode interact with the beam in a region of decreased axial beam velocity to obtain enhanced interaction with the electron beam.

Another feature of the present invention is the same as the preceding feature wherein the interaction electrode is a gate electrode with means-for applying a potential to the electrode for gating on and off the beam to obtain a switch tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein the beam focus structure includes a means for providing a second reversal of the axial beam-focusing magnetic field downstream of the first reversal and a collector electrode downstream of the second field reversal for collecting the beam.

Another feature of the present invention is the same as the first feature wherein the interaction electrode is a slow-wave circuit for obtaining enhanced slow-wave interaction with the electron beam.

Another feature of the present invention is the same as the first feature wherein the interaction electrode means is a cavity resonator to obtain enhanced interaction between the fields of the cavity resonator and the electron beam.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of a second interaction electrode disposed along the beam path in a region of a second magnetic field reversal for interaction with the electron beam downstream of the first electrode, whereby modulation superimposed on the beam by the first electrode is extracted from the beam via the second interaction electrode.

Another feature of the present invention is the same as the preceding feature wherein the first and second electrodes are either slow-wave circuits or cavity resonators, whereby improved high frequency tubes are obtained.

Another feature of the present invention is the same as the first feature wherein the interaction electrode is a repeller electrode for reversing the direction of the beam and including the provision of a cavity resonator upstream of the field reversal for velocity modulating the beam and for extracting wave energy from the modulated beam upon the second traverse of the cavity by the reversed beam, whereby an improved reflex klystron is obtained.

Another feature of the present invention is the same as the first feature wherein a pair of RF electrodes are provided upstream and downstream, respectively, of the first-mentioned interaction electrode for applying RF signal energy to the electron beam and for extracting the RF energy from the modulated beam after traverse of the first radiofrequency interaction electrode, whereby a control potential applied to the first-mentioned interaction electrode serves to control the delay time between the input and output RF signals, whereby an improved delay tube is obtained.

Another feature of the present invention is the same as the preceding feature including the provision of a pair of gate electrodes, one disposed at each end of the first-mentioned interaction electrode, to further control the delay timeof the delay tube by trapping the electron .beam in the drift space between the gate electrodes.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional view of a magnetron-injected beam tube employing features of the present invention,

FIG. 2 is a plot of axial magnetic field intensity B, as a function of the distance 2 along the axis of the beam tube of FIG. 1

FIG. 3 is an enlarged sectional view of the structure FIG. I taken along line 33 in the direction of the arrows,

FIG. 4 is a view of an alternative embodiment for that portion of the structure of FIG. 1 delineated by line 44,

FIG. 5 is a vector diagram depicting the axial V, and rotational E velocity components of the electron beam before the first magnetic field reversal,

FIG. 6 is a vector diagram similar to that of FIG. 5 depicting the velocity components of the beam after the first magnetic field reversal,

FIG. 7 is a view similar to that of FIG. I depicting an alternative switch tube embodiment of the present invention,

FIG. 8 is a plot of axial magnetic field intensity B, as a function of the distance Z taken along the axis of beam tube of FIG. 7,

FIG. 9 is a longitudinal schematic sectional view of an alternative switch tube of the present invention, FIG. 10 is a plot of axial magnetic field intensity B, as a function of distance Z taken along the axis of the tube and depicting the field reversals in the tube of FIG. 9,

FIG. I] is a plot of axial beam velocity V, versus distance Z in the axial direction for the tube of FIG. 9.

FIG. 12 is a fragmentary schematic sectional view of a traveling wave tube incorporating features of the present invention,

FIG. 13 is a plot of axial magnetic field intensity B, as a function of the axial distance Z for the tube of FIG. 12,

FIG. 14 is a plot of axial beam voltage V, as a function of the distance Z taken along the axis of the tube of FIG. 12,

FIG. 15 is an alternative embodiment for a portion of the structure of FIG. 12 delineated by line 15-15 and depicting a klystron amplifier embodiment of the present invention,

FIG. 16 is a view similar to that of FIG. I depicting a radiofrequency class C amplifier tube embodiment of the present invention,

FIG. I7 is a plot of axial magnetic field intensity B, as a function of the distance Z taken along the axis of the tube of FIG. 16,

FIG. 18 is a fragmentary view similar to that of FIG. 12 depicting an alternative traveling wave tube embodiment of the function of distance Z in the axial direction for the tube of FIG. 18,

FIG. 20 is a plot of equivalent axial beam voltage V, corresponding to the axial beam velocity as a function of the distance 2 taken along the axis of tube of FIG. 18,

FIG. 21 is a plot of gain versus frequency depicting the output characteristics for the tube of FIG. 18,

FIG. 22 is a plot of phase velocity versus frequency for the slow-wave circuit of the tube of FIG. 18 and showing beam voltage corresponding to two beam velocities,

FIG. 23 is a longitudinal sectional view of a reflex klystron embodying features of the present invention,

FIG. 24 is a plot of axial magnetic field intensity B as a function of axial distance Z taken along the axis of the tube of FIG. 23,

FIG. 25 is a longitudinal schematic sectional view of a radiofrequency delay tube incorporating features of the present invention,

FIG. 26 is a plot of axial magnetic field intensity B asa function of the axial distance taken along the axis of the tube of FIG. 25,

FIG. 27 is a view similar to that of FIG. 25 depicting an alternative embodiment of the delay tube of FIG. 25, and

FIG. 28 is a plot of axialmagnetic field intensity B as a function of the axial distance Z for the tube of FIG. 27.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a magnetron injection beam tube 1 incorporating the field reversal magnetic field focusing feature of the present invention. More particularly, the electron tube 1 includes a magnetron injection gun 2 disposed at one end of the tube 1 and a beam collector structure 3 disposed at the other end of the tube 1. The magnetron injector gun 2 includes a cathode having a cylindrical cathode emitter portion 4 disposed on the axis of the beam tube 1. The cathode also includes an outwardly flared repeller portion 5 for providing an axial component of velocity to the electrons emitted from the cathode emitter portion 4. The gun 2 also includes a cylindrical anode electrode 6 disposed surrounding the cathode electrode. The anode electrode 6 includes a cylindrical portion 7 and an inwardly tapered portion 8 for shaping the electric field lines E in the region between the cathode structure and the anode 6.

As is well known from magnetron injection gun technology, exemplified by U.S. Pat. No. 3,258,626, the gun 2 is disposed in an axially directed magnetic field B which is parallel to the cathode emitter surface 4 such as not to intercept the emitting surface at any portion thereof. The axial magnetic field is produced by a beam focus magnet structure 9 dispose surrounding the envelope of tube 1. The magnet structure 9 includes a plurality of annular axially spaced pole pieces 11, 12 and 13 disposed along the longitudinal axis of the tube 1. The pole pieces are energized by magnets 14 and 15 which may be electromagnets or permanent magnets such magnets being energized in such a way as to produce magnetic fields of opposite polarity in the regions between pole pieces 11 and 12 and 12 and 13, respectively. More specifically, the axial component of the magnetic field B as shown in FIG. 2 as a function of the distance Z taken along the axis of the tube 1. As seen from FIG. 2, the magnetic beam focusing field has a mag nitude +B in the region between pole piece 11 and 12 and a magnitude of B,,, or merely reversed, in the region between pole piece 12 and pole piece I3. Thus, the magnetic field has a field reversal at pole piece 12.

Electrons emitted from the cathode emitter 4 form a hollow beam 16 of annular cross section as shown in FIG. 3. The beam has an axial velocity corresponding to a certain axial beam voltage component and a certain rotational velocity Kg corresponding to an equivalent rotational beam voltage to produce a certain resultant beam velocity corresponding to a beam voltage V as shown in FIG. 5. When the beam passes through the magnetic field reversal at pole piece 12, the total beam velocity V,remains constant but a very substantial proportion of the beam velocity V,is transferred from an axial beam velocity V,to a rotational beam velocity Y; such that the axial velocity V of the beam is rapidly diminished to a much smaller value than before the field reversal and the rotational velocity-B of the beam is substantially increased, as shown in the vector diagram of FIG. 6. In a preferred embodiment, the envelope portion of the tube is operated at anode potential as is the collector electrode 3 and the cathode structure is operated at a corresponding to the total beam velocity V The cathode emitter 4 is carried from an insulator I 17 forming one end of the vacuum envelope, such insulator l7 sealing off the end of the tube with a lead passing from a source beam voltage :Q to the cathode emitter through the insulator 17.

Referring now to FIG. 4, there is shown an alternative magnetron injection gun 2. In this gun 2, the cathode emitter por tion 4 is not a right circular cylinder but is the outer surface of a frustium of a cone and the axial magnetic field B is arranged to be parallel to the conical surface of the emitter 4 such is not to intercept the emitting surface 4. The conical-shaped anode electrode 7 is arranged to coaxially surround the cathode emitter 4 to provide a generally radially directed electric field in the region between the anode and the cathode. The magnetron injection gun of FIG. 4 operates generally in the same manner as the gun 2, previously described with regard to FIG. 1.

Referring now to FIGS. 7 and 8, there is shown a switch tube 21 incorporating the field reversal features of the present invention. More particularly, the tube 21 is essentially the same as that of FIG. 1 with the exception that a tubular control electrode 22 is disposed surrounding the beam 16 in the region of the tube envelope downstream of the first magnetic field reversal such that the control electrode 22 is in the field reversed region where the axial velocity of the beam has been greatly reduced. In this region, a relatively small control voltage applied to the control electrode 22 can be employed for gating on" and oft the axial flow of electrons along the beam path from the magnetron injection gun 2 to the collector 3. The gate voltage is supplied from a gate voltage supply 23 to the control electrode 22, for gating on and off the beam, thereby forming a switch tube. The advantage of the switch tube 21 is that a relatively small control voltage may be applied to the control electrode 22' for controlling the beam current. The p. of the control electrode 22 is the ratio of the axial beam velocity before field reversal divided by the axial beam velocity after field reversal, as indicated in FIG. 11.

Referring now to FIG. 9 there is shown an alternative switch tube embodiment of the present invention. This embodiment is substantially the same as that of FIG. 7 with the exception that a second field reversal is provided following the control electrode 22 in the region of the collector 3. The second field reversal is as indicated in FIG. 10 and is provided by a third magnet 24 oppositely polarized to magnet 15 and including an annular pole piece 25. The axial beam velocity V versus axial distance Z along the beam path is as shown in FIG. 11. In this embodiment, the beam 16 is collected at full axial beam velocity V,. However, as in the embodiment of FIG. 7, the p of the control electrode 22 is substantially the same as that of FIG. 7.

Referring now to FIGS. 12-14, there is shown a microwave traveling wave tube 27 incorporating features of the presentinvention. The structure of FIG. 27 is substantially the same as that previously described with regard to FIG. 9 with the exception that the control electrode 22 is replaced by a slow-wave interaction circuit structure 28 and 32. More particularly. a first slow-wave structure 28, such as a helix ring and bar or other type of slow-wave structure is arranged for interaction with the hollow beam 16, and is located in the region of the beam following the first field reversal.

The slow-wave circuit circuit 28 is terminated at the downstream end via a resistive termination 29 and includes an input terminal 31 at the upstream end of the circuit for receiving RF energy to be amplified. The microwave slow-wave circuit structure also includes a second slow-wave circuit portion 32 having a portion disposed within the region of the first field reversal and a second portion disposed within the region of the second field reversal. The upstream end of the second slowwave circuit 32 is terminated in a resistive termination 33 and the downstream end is terminated in output terminal 34 for extracting output RF energy from the slow-wave circuit 32. The axial beam velocity V taken along the axis of the tube 27 is shown in FIG. 14. It can be seen that in the region of the first field reversal the axial beam velocity V is substantially half of the axial beam velocity 2V in the region of the gun. Accordingly, the first slow-wave circuit 28 is dimensioned to have a synchronous phase velocity at the reduced axial beam velocity V,. The second slow-wave circuit 32 is also dimensioned to have a synchronous phase velocity at the lower axial beam velocity of V The circuit is arranged such that as the interaction between the signal coupled onto the upstream end of the second slow-wave circuit and the beam begins to reach saturation, the second field reversal is reached corresponding to the position of pole piece 13. At this point, the axial beam velocity is abruptly increased to a value such as 2V, thereby reducing the beam perveance to the original value before the first field reversal. This will increase the efficiency of the tube because the slow electron bunches, which were about to move into an accelerating phase of the wave on the circuit now begin to move forward rapidly in the decelerating phase thereby giving up substantial additional energy to the wave.

Thus, in this embodiment, RF energy is put onto the first slow-wave circuit 28 for producing velocity modulation of the beam and cumulative interaction to produce a growing amplified wave on the first section. The wave on the first section is terminated in resistor 29 and the bunched beam carries the signal modulation onto the second slow-wave circuit 32 in the manner of the conventional severed circuit tubes. The modulation of the beam induces a growing wave on the second slowwave circuit 32 which reaches near saturation at the second field reversal corresponding to the position of pole piece 13 and at this point an abrupt increase in the axial velocity of the beam is obtained to produce increased efficiency for the con version of microwave energy from the bunches in the beam to the slow-wave circuit. The wave continues to grow on the second slow-wave circuit portion 32 and is coupled from the tube via output terminal 34 to a suitable load, not shown.

Although the tube 27 of FIG. 12 has been shown employing helix-type slow-wave circuit this is not a requirement. Other types of slow-wave circuits 28 and 32 may be employed such as ring and bar, coupled cavity, contrawound helices, ring and loop and various other similar circuits.

Another advantage of the tube of FIG. 12 is that most of the gain for the tube 27 is obtained in the region between the pole pieces 12 and 13, where beam perveance is high and the axial beam velocity is low, thus, achieving very high gain per unit length. On the other hand, power generation is obtained in the region between pole pieces 13 and 25 where axial beam velocity is high and the perveance low, thus, achieving high efficiency. The net result is a relatively short-traveling wave tube for the gain and power produced.

Referring now to FIG. 15, there is shown a klystron amplifier tube embodiment similar to the traveling wave tube of FIG. 12. More specifically, the axial magnetic field reversals are the same as that of FIG. 12 and the traveling wave slow-wave circuits 28 and 32 of FIG. 12 have been replaced by a succession of cavity resonators. An input cavity resonator 36 and a first buncher cavity 37 are disposed in the relatively low beam velocity region between pole pieces I2 and 13 such that the input cavity 36 and first buncher cavity 37 operate in a region of relatively low beam velocity and relatively high perveance, whereas a penultimate cavity 38 and output cavity 39 are disposed in the region of the second magnetic field reversal between pole pieces 13 and 25 such that the penultimate cavity 38 and output resonator 39 operate in a region of relatively low beam perveance and relatively 'high beam velocity. Input wave energy to be amplified is applied to the input resonator 36 for velocity modulating the beam in the relatively low beam velocity region and output amplified microwave energy is extracted from the output cavity 39 which extracts the wave energy from the modulated beam passing to the collector 3. As in the traveling wave tube embodiment of FIG. 12, the klystron of FIG. 15 provides for a reduced overall tube length for a given frequency due to the reduction of the beam velocity in the driver section of the tube where the tube is operating in a relatively small signal regime, thereby provided a relatively high gain per wavelength to minimize overall tube length.

Referring now to FIGS. 16 and 17, there is shown a class C amplifier incorporating features of the present invention. The structure of the tube of FIG. 16 is substantially the same as that of FIG. 15 with the exception that the tube includes only an input resonator 36 disposed in the high-perveance low beam velocity region of the first beam reversal and a single output cavity 39 is disposed in the high-beam voltage, low beam perveance region of the second field reversal. The input cavity 36 passes beam current on the positive half cycles of the RF in the cavity 36 and reflects beam current on the negative half cycles of the RF. The reflected beam current will return to the cathode where it will be reflected. The pulses of beam current, upon passage through the second magnetic field reversal occasioned by pole piece 13, will be accelerated to relatively high axial velocity and serve to excite resonance of the output resonator 39. The achievable power gain for the class C amplifier of FIG. 16 is about equal to the square of the ratio of the axial beam velocity in the forward field region to the axial beam velocity in the reverse field region. A gain of between 20 to 30 db. should be realizable. FIG. 17 shows the magnetic field reversals as a function of the axial distance within the tube.

Referring now to FIGS. 18 through 22 there is shown an alternative traveling wave tube structure 41, similar to that previously described with regard to FIG. 12 which employs alternative features of the present invention to provide increased bandwidth. More particularly, the tube 4i is essentially the same as that of FIG. 12 with the exception that a slow-wave circuit 42 that is employed is dispersive having a phase velocity versus frequency characteristic as shown in FIG. 22. Typical examples of such dispersive circuits include the ring and loop or ring and bar circuit. The region of the first reversal of the magnetic field between pole pieces 12 and 13 produces a substantial reduction in the axial beam velocity from 2V to V The higher beam velocity 2V produces increased interaction with the slow-wave circuit 42, at the low frequency end of the operating band, indicated at f, in FIGS. 21 and 22. On the other hand, the low beam velocity section provides increased gain and interaction with the circuit at the upper frequency edge of the operating band, namely f,,. as shown in FIGS. 21 and 22. The composite gain for the total interaction is shown by the dotted line of FIG. 21 and it is seen that the composite gain curve provides substantial gain over a band from below f, to above f An advantage to the tube 41 of FIG. 18 is that by the use of a dispersive circuit the second harmonic output is substantially reduced as compared to the use of a nondispersive slowwave circuit 42. The reason for this is that when the tube is driven at the low frequency end of the operating band, the RF wave and the beam fail to synchronize in the input of the tube where the beam axial velocity is low. However, in the output half, where the beam velocity is high, the drive frequency synchronizes with the beam but the second harmonic of the drive frequency is removed from synchronism, hence, it does not interact with the beam. Another advantage of the tube of FIG. [8 is that high beam perveance interaction is obtained in the low-velocity portion of the tube which corresponds with the high frequency end of the operating band where the impedance of the circuit is lowest and the dispersion the greatest.

Thus, the high perveance, low beam impedance section will help to broaden the band and maintain the gain per wavelength in this region. Higher gain tubes of the type shown in FIG. 18 can be obtained by cascading slow-wave circuit sections with circuit severs between the sections as previously indicated with regard to FIG. 12. As an alternative, a helix which is a low-dispersive circuit may be heavily loaded to avoid occilation and employed as the input slow-wave circuit section.

Referring now to FIGS. 23 and 24, there is shown a reflex klystron 45 incorporating features of the present invention. The tube 45 is similar to that depicted in FIG. 7 with the exception that the interaction electrode 22 is replaced by a reflector electrode 46 energized with a suitable negative potential as applied from a repeller voltage supply 47. An input cavity resonator 48 is disposed in the high-velocity region of the beam before the first field reversal produced by pole piece 12 for velocity modulating the beam 16 with RF energy at the resonant frequency of the cavity. The cavity is excited into resonance by noise energy on the electron stream. By disposing the reflector electrode 46 in the high-perveance low axial beam velocity region, a relatively low repeller voltage may be employed for reflecting the beam. Moreover, relatively large transittimes may be obtained for the beam passing through the low-velocity region to facilitate construction of low frequency reflex klystron tubes with relatively short overall length.

Referring now to FIGS. 25, and 26 there is shown a novel RF delay tube incorporating features of the present invention. The delay tube 51 of FIG. 25 is essentially the same as the switch tube of FIG. 9 with the exception that a delay voltage supply 52 supplies only sufficient voltage to delay electrode 22 to substantially reduce the already low axial velocity of the beam 16 in the high beam perveance low axial beam velocity region of the first beam reversal between polepieces 12 and 13. In addition, an input RF circuit 53 such as a helix is placed upstream of the first field reversal and a second output RF circuit such as a helix 54 is disposed in the region of the second magnetic field reversal between pole pieces 13 and 25.

In operation, RF energy which it is desired to delay is applied to the beam 16 in the region of the first RF circuit 53 which is terminated at its downstream end in resistor 55. The modulated beam then passes into the region of the delay electrode 22 wherein its axial velocity is slowed due to the field reversal and further slowed due to the retarding potential applied to the delay electrode 22. Upon passage through the delay electrode 22 the modulated beam passes into the region of the second field reversal where the axial beam velocity is substantially increased with a corresponding lowering of beam perveance.

In the region of the second field reversal, the RF energy on the beam is coupled onto the output circuit 54 and fed to a suitable utilization device, not shown. The input end of the output circuit 54 is terminated in a resistive termination 56. A small time varying voltage applied to delay electrode 22 can be used to vary the phase, or frequency or time delay of the output signal relative to the input signal. A sawtooth voltage will vary the output frequency relative to the input frequency. This is done in conventional traveling wave tubes by applying a sawtooth voltage to the helix and is called serrodyning, but requires a much higher voltage than is required in the case of the tube of FIG. 25. A variable phase or time delay is obtained by programming the voltage applied to the control electrode 22 to achieve the desired result.

Referring now to FIGS. 27 and 28 there is shown an altemative RF delay tube 58 incorporating features of the present invention. Delay tube 58 is essentially the same as delay tube 51 of FIG. 25 with the exception that a pair of ring-shaped gate electrodes 59 and 61, respectively, are disposed at the ends of the delay electrode 22. Such gate electrodes 59 and 61 are disposed within the first field reversal region between pole pieces 12 and 13. A pair of voltage supplies 62 and 63 are provided for applying gate voltages to gate electrodes 59 and 61 relative to the voltage applied to the drift electrode 22. In operation, the downstream gate 61 is initially closed by applying a few volts negative to gate electrode 61 relative to the potential applied to the drift electrode 22. A predetennined time after the beam is turned on, the first gate electrode 59 is closed by applying a sufficiently negative voltage to the gate electrode 59 from the first gate potential supply 62 to reflect the beam back to the cathode. The predetermined time delay before closing the first gate is at least as long as the time required for an electron to travel the length of the drift region to the second gate 61 and return to the first gate 59. This typically is on the order of a few hundred nanoseconds. The closing of the first gate 59 traps the modulate electron beam, which oscillates between the two gates at the reduced axial beam velocity dictated by the field reversal and the potential applied to the drift electrode 22. When the desired elapsed time has passed, the downstream gate 61 is opened thereby emptying the stored beam through the output circuit 54 where the stored RF signal is coupled from the beam onto the circuit for transmission to a suitable load, not shown. The advantage of the delay tube 58 of FIG. 27 is that relatively long delay times can be obtained.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: i

1. In an electron tube apparatus, a hollow anode electrode structure, a cathode emitter structure having an electronemissive portion coaxially disposed within said anode electrode, means for applying an electrical potential between said anode and cathode electrodes to establish a generally radially directed electric field in the region between said anode and said electron-emissive portion of said cathode electrode, magnetic beam focus means for establishing a generally axially directed beam focus magnetic field over a predetermined beam path and in the region between said anode and said electron-emissive portion of said cathode structure, such beamfocusing magnetic field being parallel to said electron-emis sive surface of said cathode emitter such as not to intercept the emitting surface, means for projecting electrons emitted from said cathode emitter into a generally axially directed hollow beam which is directed generally parallel to the axial directed beam focus magnetic field over the predetennined beam path, the improvement wherein, said magnetic beam focus means includes means for reversing the axial direction of the beam focus magnetic field to define a reversed field region of magnetic beam-focusing field in the beam path downstream of the field reversal and to produce in the reversed field region of the beam path a reduction of the axial velocity of the hollow beam with an accompanying corresponding increase in rotational velocity of the beam, and interaction electrode means disposed within said reversed field region of the beam path for electronic interaction with the beam in the reversed field region of decreased axial beam velocity, whereby enhanced electronic interaction between the beam and the electric field of said interaction electrode is obtained for a given electrical potential established on said interaction electrode and funher including means downstream of said interaction electrode for collecting the beam passable by said interaction electrode, and means for applying a gating po en ia to said interaction electrode for gating on and off the beam passable by said interaction electrode, whereby a switch tube is obtained.

2. The apparatus of claim 1 including means for producing a second reversal of the axial magnetic beam focus field in the beam path downstream of said first field reversal to define a second magnetic field-reversed region of the beam path, and means disposed downstream of said second field reversal for collecting the beam.

3. The apparatus of claim I wherein said interaction electrode means is a slow-wave circuit.

4. The apparatus of claim f wherein said interaction electrode means is a cavity resonator.

5. The apparatus of claim 2 including a second interaction region, said second and third radiofrequency electrodes being provided for modulating the beam with radiofrequency energy and for extracting the radiofrequency energy from the modulated beam, respectively, and wherein said first interaction electrode includes means for applying a control potential for controlling the delay time between modulation of the beam with the RF energy and extraction of that RF energy from the beam downstream of said control electrode.

9. The apparatus of claim 8 including a pair of gate electrodes one disposed at the beam entrance and one disposed at the beam exit, respectively, of said first interaction electrode means to further control the delay time. 

1. In an electron tube apparatus, a hollow anode electrode structure, a cathode emitter structure having an electronemissive portion coaxially disposed within said anode electrode, means for applying an electrical potential between said anode and cathode electrodes to establish a generally radially directed electric field in the region between said anode and said electron-emissive portion of said cathode electrode, magnetic beam focus means for establishing a generally axially directed beam focus magnetic field over a predetermined beam path and in the region between said anode and said electron-emissive portion of said cathode structure, such beam-focusing magnetic field being parallel to said electron-emissive surface of said cathode emitter such as not to intercept the emitting surface, means for projecting electrons emitted from said cathode emitter into a generally axially directed hollow beam which is directed generally parallel to the axial directed beam focus magnetic field over the predetermined beam path, the improvement wherein, said magnetic beam focus means includes means for reversing the axial direction of the beam focus magnetic field to define a reversed field region of magnetic beam-focusing field in the beam path downstream of the field reversal and to produce in the reversed field region of the beam path a reduction of the axial velocity of the hollow beam with an accompanying corresponding increase in rotational velocity of the beam, and interaction electrode means disposed within said reversed field region of the beam path for electronic interaction with the beam in the reversed field region of decreased axial beam velocity, whereby enhanced electronic interaction between the beam and the electric field of said interaction electrode is obtained for a given electrical potential established on said interaction electrode and further including means downstream of said interaction electrode for collecting the beam passable by said interaction electrode, and means for applying a gating potential to said interaction electrode for gating on and off the beam passable by said interaction electrode, whereby a switch tube is obtained.
 2. The apparatus of claim 1 including means for producing a second reversal of the axial magnetic beam focus field in the beam path downstream of said first field reversal to define a second magnetic field-reversed region of the beam path, and means disposed downstream of said second field reversal for collecting the beam.
 3. The apparatus of claim 1 wherein said interaction electrode means is a slow-wave circuit.
 4. The apparatus of claim 1 wherein said interaction electrode means is a cavity resonator.
 5. The apparatus of claim 2 including a second interaction electrode means disposed along said beam path in saId second field-reversed region for interaction with the electron beam downstream of said first electrode means.
 6. The apparatus of claim 5 wherein said first and second interaction electrode means are slow-wave circuits.
 7. The apparatus of claim 5 wherein said first and second interaction electrodes means are cavity resonators.
 8. The apparatus of claim 2 including second and third radiofrequency electrode means, said second electrode means being disposed upstream of said field reversal and said third electrode means being disposed in said second field-reversed region, said second and third radiofrequency electrodes being provided for modulating the beam with radiofrequency energy and for extracting the radiofrequency energy from the modulated beam, respectively, and wherein said first interaction electrode includes means for applying a control potential for controlling the delay time between modulation of the beam with the RF energy and extraction of that RF energy from the beam downstream of said control electrode.
 9. The apparatus of claim 8 including a pair of gate electrodes one disposed at the beam entrance and one disposed at the beam exit, respectively, of said first interaction electrode means to further control the delay time. 